The present invention relates to polymer bonded energetic materials.
Polymer bonded energetic materials comprising an energetic filler material, usually in the form of a solid crystalline powder, formed into a consolidated mass having suitable mechanical properties and insensitivity by a polymeric binder are well known and are used in a variety of military and civilian applications. Such materials in various compositions are used for example as high explosives for use in demolition, welding, detonating, cutting charges and munition fillings, as propellants for guns and rockets, as gas generators and as pyrotechnics.
Binders used in polymer bonded energetic materials need to be (amongst other things) compatible with the other ingredients of the material and suitably processed together with the other ingredients into the appropriate shapes required in the various applications.
Polymeric binders may be classified generally into chemically cured materials and thermoplastic materials. Chemically cured materials, eg. thermosetting resins, rely on the chemical reaction between different components to provide the desired polymeric structure.
The reacting components are normally brought together during manufacture of the end product material, eg. when the material is shaped, eg. cast, moulded or extruded. The cure time can be lengthy, and hence costly, and it can be difficult to control the chemical reaction involved.
Thermoplastic binders allow energetic materials containing them to be processed at elevated temperatures, usually outside the in-service envelope of the end product, but cool to give dimensionally stable sheet, bars, cylinders and other shapes. Shaping of the end product relies on purely physical changes taking place in the binder of the material. Reject materials may be re-cycled by re-heating. This may not normally be achieved with materials based on chemically cured binders.
The use of thermoplastic binders in known energetic materials has shown disadvantages in each case.
For example, a known material described in UK Patent No.1,082,641 herein called xe2x80x9cComposition Axe2x80x9d comprising RDX (1,3,5-cyclotrimethylene-2,4,6-trinitramine) as energetic filler and a mixture of polyisobutylene, di-(2-ethylhexyl)sebacate and polytetrafluoroethylene as thermoplastic binder is used as a conventional service material in a number of military applications as a plastic bonded high explosive but this material suffers from the problems (a) that it is difficult to shape under pressure, eg by extrusion, (b) when rolled into sheets it has anisotropic properties, and (c) when deformed it has little elastic memory to regain its original shape.
It is known to produce polymer bonded energetic materials such as solid explosives and propellants using an ethylene-vinyl acetate (EVA) copolymer as a thermoplastic binder. UK Patent Specification No.1,554,636 describes for use in explosive compositions EVA copolymers which are mixed with a plasticiser in order to reduce the temperatures at which the binder may be processed.
We have discovered however that EVA copolymers modified in the manner described in UKP 1,554,636 are not ideal in a number of respects, particularly as regards their mechanical properties, for use in polymer bonded energetic materials such as explosives.
It is the purpose of the present invention to provide a novel thermoplastic polymer bonded energetic material in which the polymer binder is specially selected to overcome the problems shown in the prior art by known thermoplastic polymer bonded energetic materials.
According to the present invention a thermoplastic polymer bonded energetic material comprises a composition which comprises:
Component A: an energetic filler material; and
Component B: a polymeric binder for the energetic filler material;
wherein the ratio of the weight of Component A present to the weight of Component B present in the composition is in the inclusive range from 1:10 to 199:1 and wherein Component B comprises an intimate mixture of Ingredients 1 and 2 as follows:
Ingredient 1: a copolymer of ethylene and vinyl acetate;
Ingredient 2: a copolymer of butadiene and acrylonitrile; the ratio of the weight of Ingredient 1 present to the weight of Ingredient 2 present in Component B being in the inclusive range from 1:10 to 10:1.
Ingredients 1 and 2 will be referred to herein as xe2x80x9cEVAxe2x80x9d and xe2x80x9cBNxe2x80x9d respectively. The terms xe2x80x9cEVIAxe2x80x9d and xe2x80x9cBNxe2x80x9d will herein be understood to include compounds in which other units are optionally copolymerised with the ethylene and vinyl acetate units on the one hand and the butadiene and acrylonitrile units on the other hand. These terms will also be understood to include coplymers containing optional substituents, eg. halides or methyl groupings, in the ethylene, vinyl acetate, butadiene and acrylonitrile units.
Preferably, the softening point of Component B is greater than 60xc2x0 C. desirably greater than 80xc2x0 C.
Preferably, the BN per se (prior to introduction to the other components) is in the form of a liquid having a viscosity greater than 50 cst when measured at a temperature of 20xc2x0 C. and a molecular weight in the range 200 to 20,000, desirably in the inclusive range 2000 to 5000. Such a compound may be modified in the course of processing to form a product.
The material according to the present invention may, for example, be in the form of a consolidated rubbery mass, the energetic filler Component A preferably being a particulate, eg. powdered, solid, being embedded in the binder Component B.
The polymer bonded energetic materials according to the present invention give mechanical properties superior to those of the prior art materials described in UKP 1,554,636. The plasticisers employed in the polymer bonded explosive compositions described in UKP 1,554,636 are generally non-viscous mobile liquids of viscosity less than 50 cst, typically 10 cst at 20xc2x0 C. which can exude from the compositions containing them during temperature cycling in storage or use. This causes the composition to become brittle with age. Furthermore, the said plasticisers do not give satisfactory adhesion to the explosive material and this can result in useless crumbly material at some plasticiser concentrations.
In contrast, the BN polymers employed in the compositions according to the present invention to plasticise the EVA do not substantially migrate during storage or use and give good adhesion to the energetic filler material as well as to the EVA and this provides compositions having improved physical, mechanical and ageing properties.
The materials according to the present invention can show improvements over the materials of UKP 1,082,641 in that they have properties which are substantially isotropic and may be formed more easily into desired shapes, such as by rolling, pressing, moulding, extruding or casting, which can retain their elastic memory and repair their shape when deformed.
In Component B of the materials according to the present invention, optional additives may be included in the mixture together, with EVA and BN. Examples of such additives include plasticisers and antioxidants. Examples of suitable optional additives are given hereinafter.
Preferably, the optional additives will comprise in total not more than 20 percent by weight, normally less than 10 percent by weight, of Component B.
Component B may comprise from 25 to 85, preferably 50 to 75 percent, by weight EVA and from 20 to 60, preferably 25 to 50, percent weight BN.
The EVA present in Component B may have a vinyl acetate content of from 25 percent to 75 percent, desirably from 33 to 60 percent inclusive, especially 40 to 45 percent inclusive. This polymer may be provided in the form of a mixture of different EVA compounds having different vinyl acetate contents.
An EVA copolymer containing 45 percent by weight vinyl acetate has been shown to provide a particularly satisfactory example.
The BN present in Component B may have a bound acrylonitrile content in the inclusive range 5 to 50 percent by weight, desirably in the inclusive range 10 to 30 percent by weight. The BN polymer contained in component B may be provided by a mixture of different BN compounds, eg. having a different acrylonitrile content.
The BN polymer or polymers included in Component B may have functional terminations. For example, these polymers may be carboxyl terminated, hydroxy terminated, amino terminated or vinyl terminated. Alternatively, the polymer may be non-functionally terminated.
Polymers comprising acrylonitrile/carboxyl terminated butadienes may include as copolymerised monomer units optionally substituted alkyl chains, eg. dimethylene optionally substituted with a carboxyl group.
Carboxyl terminated acrylonitrile/butadiene having a bound acrylonitrile content of 26 percent, a bound butadiene content of 74 percent by weight and a molecular weight of 3200 has been found to provide a particularly satisfactory example.
The energetic filler and the relative proportions of the components of the energetic material will, as will be appreciated by those versed in the art, depend upon the type of application for which the material is to be used.
The energetic material according to the present invention may for example comprise a plastic bonded explosive in which the binder forms between 0.5 and 30 percent by weight and the energetic filler forms between 99.5 and 70 percent by weight. Examples of energetic fillers which may be incorporated in such materials include organic secondary explosives. Alicylclic nitramines such as RDX (1,3,5-cyclotrimethylene-2,4,6,-trinitramine) and HMX (1,3,5,7-cyclotetramethylene-2,4,6,8-tetranitramine) and TATND (tetranitro-tetraminodecalin) and mixtures thereof are preferred for use as such organic fillers but the following highly energetic organic fillers may also be used as the main or as a subsidiary energetic component in plastic bonded explosives: nitroguanidine, aromatic nitramines such as tetryl, ethylene dinitramine, nitrate esters such as nitroglycerine, butanetriol trinitrate and PETN (pentaerythritol tetranitrate). Other nitroaromatic compounds such as trinitrotoluene (TNT) triaminobenzene (TATB) triaminotrinitro benzene (TATNB) and hexanitrostilbene may also be used. Alternatively inorganic fillers such as ammonium nitrate and alkaline earth metal salts provide suitable high explosive materials. Metallic fuels such as powdered aluminium, magnesium or zirconium may be used to fuel the exothermic reaction of the oxidation of the energetic material. The metallic fuel may comprise up to 50 percent by weight of the energetic filler.
The energetic materials according to the present invention may alternatively comprise a gun propellant. In such a material the content of the energetic filler is generally in the range 70 to 90 percent by weight of the binder/filler mixture and may be selected for example from nitroglycerine, RDX and HMX or a combination thereof, optionally with other highly energetic fillers such as those listed above. The binder of such a material may comprise in addition to the blend specified above a cellulosic material eg. nitrocellulose eg. forming from 5 to 95 percent, eg. 30 to 70 percent by weight of the binder.
The energetic material according to the present invention may alternatively comprise a gas generator material eg. for power cartridges for actuators: for base burning, reduced base drag, extended range projectiles: and for control gas jets for missile and projectile guidance systems and the like. Such material is similar in nature to a propellant, but in general contains a lower content of energetic filler, eg. 45% to 65% by weight energetic filler optionally together with a surface burning rate inhibitor, eg. ethyl cellulose.
As an example of a suitable rocket propellant embodying the invention the propellant composition may include as energetic filler ammonium perchlorate (20 to 90 percent by weight of the energetic filler) together with aluminium as fuel (5 to 50 percent by weight of its mixture with energetic filler), the binder forming for example 5 to 30 percent by weight of the composition.
The energetic material according to the present invention may also comprise a polymer bonded pyrotechnic material, eg. containing an inorganic nitrate or perchlorate of ammonium, barium or strontium (forming 20 to 80 percent by weight of the energetic filler), a metallic fuel such as magnesium or zirconium (forming 5 to 60 per cent by weight of the filler), the binder comprising 5 to 30 percent by weight of the overall composition.
Although the use of non-viscous plasticisers may be avoided by use of the polymer bonded energetic materials according to the present invention because the BN polymers have a plasticising effect upon the EVA, non-viscous plasticisers may optionally be incorporated in low concentrations in the compositions according to the present invention, eg. preferably less than 10 percent by weight of the binder formed by addition to Component B eg. to improve binder processibility.
For example, common plasticisers which are dialkyl esters of phthalic, adipic and sebacic acids may be used as the optional plasticiser, eg. the plasticiser may comprise dibutyl phthalate, disobutyl phthalate, dimethyl glycol phthalate, dioctyl adipate or dioctyl sebacate.
In addition, or alternatively, energetic plasticisers such as GAP (glycidyl azide polymer), BDNPA/F (bis-2-dinitropropylacetral/formal), bis-(2-fluoro-2, 2-dinitroethyl)formal, diethylene glycol dinitrate, glycerol trinitrate, glycol trinitrate, triethylene glycerol dinitrate, trimethylolethane trinitrate butanetriol trinitrate, or 1,2,4-butanetriol trinitrate, may be employed in concentration less than 10 percent by weight of binder formed by addition to Component B in the materials according to the present invention.
In the material according to the present invention other binder polymers may be blended with the composition provided by Component B in a concentration of up to 45 percent by weight, preferably less than 20 percent by weight of the overall binder content formed by the addition. The additional binder polymer(s) may comprise an inert binder material, an energetic binder material or a blend of inert and energetic binder materials.
Examples of suitable additional inert or non-energetic binder materials are cellulosic materials such as the esters, eg. cellulose acetate, cellulose acetate butyrate, and synthetic polymers such as polyurethanes, polyesters, polybutadienes, polyethylenes, polyvinyl acetate and blends and/or copolymers thereof.
Examples of suitable energetic binder materials are nitrocellulose, polyvinyl nitrate, nitroethylene, nitroallyl acetate, nitroethyl acrylate, nitroethyl methacrylate, trinitroethyl acrylate, dinitropropyl acrylate, C-nitropolystyrene and its derivatives, polyurethanes with aliphatic C- and N-nitro groups, polyesters made from dinitrocarboxylic acids and dinitrotrodiols and nitrated polybutadienes.
We prefer for use as additional binders especially where the material is a propellant or gas generator charge material, cellulosic materials, comprising 100 to 40 percent by weight of nitrocellulose and 10 to 60 percent by weight of an inert cellulose ester eg. cellulose acetate or cellulose acetate butyrate.
Extenders may be used as part of the binder formulation to improve the processibility and flexibility of the product. For example, heavy grade liquid paraffin (up to 3 percent by weight of the binder formulation) may be employed in the binder.
Various known minor additives may be added to the materials according to the present invention. Preferably, the additives content comprises no more than 10 percent by weight, desirably less than 5 percent by weight, of the overall energetic material composition.
For example in propellant and gas generator compositions the additive may for example comprise one or more stabilisers, eg. carbamite or PNMA (para-nitromethylaniline); and/or one or more ballistic modifiers, eg. carbon black or lead salts; and/or one or more flash suppressants, eg. one or more sodium or potassium salts, eg. sodium or potassium sulphate or bicarbonate.
Antioxidant in an extent of up to 1 percent by weight of the overall composition of the energetic materials may usefully be incorporate in the materials. A suitable antioxidant has been found to be 2,2xe2x80x2-methylene-bis(4-methyl-6-butyl)phenol.
Coupling agents known per se, eg. in concentrations of up to 2 percent by weight of the overall composition weight, may be employed to improved adhesion between the binder and energetic filler components (Components A and B) in the materials according to the present invention.
Preferably, where the energetic material according to the present invention is a plastic bonded explosive it contains the following components (in percentage parts by weight):
RDX: 80-99.5 percent, preferably about 88 percent;
binder: 20-0.5 percent, preferably about 12 percent;
Preferably the binder comprises 60 to 75 percent EVA (preferably at least 25 percent of which is a polymer having a 45% by weight vinyl acetate content); 25 to 50 percent BN, and 0 to 1 percent antioxidant, the overall percentages (excluding further optional additives) adding to 100 in each case.
Compositions which are materials according to the present invention may be processed into manufactured products by processes which are generally known per se. For example, for the manufacture of plastic bonded explosives the binder ingredients may be added and mixed together in a blender at temperatures of 80xc2x0 C. to 140xc2x0 C. and then added to the energetic filler by a solventless process or a solvent lacquer process.
In a solvent lacquer process, for example, the binder is dissolved in an organic solvent at a moderately elevated temperature, eg. 40xc2x0 C. to 80xc2x0 C. and the energetic filler is subsequently stirred into the solvent lacquer after cooling to about 20xc2x0 C. to give a slurry. The slurry is then mixed under vacuum at an elevated temperature, eg. 50xc2x0 C. to 90xc2x0 C., preferably 75xc2x0 C. to 90xc2x0 C. In a solventless process for example, for the production of plastic bonded nitramines the required quantity of pre-dried energetic filler material is wetted with water or an aqueous solution and heated to an elevated temperature, eg. 80xc2x0 C.-100xc2x0 C. The binder is then added to the energetic filler and the components are mixed together at that temperature. Any water remaining in the composition is removed under vacuum. Materials produced by these different routes give no discernible difference in properties.
Materials produced in the ways described above or in other known ways may, depending on the material composition and its intended use, may be shaped into products in known ways. For example, the material may be pressed, moulded or cast into a desired shape eg. for use as blocks, sheet explosive or for filling of shells, warheads and the like. Alternatively, the material may be extruded in a known manner in a corotating twin screw extruder, and subsequently cooled. The latter technique is especially suitable for the manufacture of gun propellant materials, eg. stick or tubular propellants of known cross-sectional shape.
In summary, the energetic materials of the present invention may depending upon their specific composition and properties be used in any one or more of the following well known applications:
(i) General demolition;
(ii) Explosive welding;
(iii) Active armour;
(iv) Detonating cord;
(v) Linear cutting charges;
(vi) Shell fillings;
(vii) Mine fillings;
(viii) Grenade fillings;
(ix) Shaped-charge warhead fillings;
(x) Fragmentation warhead fillings;
(xi) Booster pellets;
(xii) Peripheral initiation and detonation transfer systems;
(xiii) Gun propellants;
(xiv) Rocket propellants;
(xv) Gas generators;
(xvi) Pyrotechnics;